<!DOCTYPE html>
<html>
  <head>
    <style>
      div.last{clear:left;}
      div{background:#ccc;height:200px;width:200px;border:3px solid blue;float:left;}
    </style>
  </head>
  <body>
    <!--切记此处因为是变量,因此一定要加=-->
    <div class="last">0</div>
    <div>1</div>
    <div>2</div>
    <div>3</div>
    <!--切记此处因为是变量,因此一定要加=-->
    <div class="last">4</div>
    <div>5</div>
    <div>6</div>
    <div>7</div>
    <!--切记此处因为是变量,因此一定要加=-->
    <div class="last">8</div>
    <div>9</div>
    <div>10</div>
    <div>11</div>
    <div>12</div>
  </body>
</html>